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Combined effect of soil moisture and external loads on soil compaction

MA Hamza1, SS Al-Adawi2 and KA Al-Hinai2

1 Department of Agriculture and Food, Western Australia, 444 Albany Highway, Albany, WA 6330, Australia,
2
Dept. of Soils, Water and Agricultural Engineering, PO Box 34 AlKhoud,Sultan Qaboos University, Muscat, Oman,
Corresponding author. Email: mhamza@agric.wa.gov.au.

Abstract

This experiment was designed to study the influence of combinations of external load (tractor weight) and soil water on soil compaction. Four soil water levels: air dry, 50% FC, FC and saturation were combined with four external loads: no load (0 kg), small tractor (2638 kg), medium tractor (3912 kg) and large tractor (6964 kg). Soil bulk density, soil strength and soil water infiltration rate were measured at 0-100, 100-200 and 200-300 mm soil depths. Combined increases in soil water and external load together increased soil compaction, as indicated by increasing soil bulk density and soil strength and decreasing soil water infiltration rate. However, at soil water below field capacity, the soil compaction increases were not significant for any external load. The interaction between soil water and external load was not significant for bulk density for all soil depths but was significant for soil strength and soil water infiltration rate for all soil depths.

Introduction

Soil compaction is an important component of the land degradation syndrome, which is an issue for soil management throughout the world (Batey 2009). Soil water content is the most important factor influencing soil compaction processes (Soane and Van Ouwerkerk 1994; Hamza and Anderson 2003, 2005). However, uncertainty still exists regarding the effect of soil water on the susceptibility to compaction. Contradictory results have been reported in the literature. This paper is designed to study such an effect.

Materials and methods

Site description

The experiment was located at the Agricultural Experiment Station at Sultan Qaboos University, Oman (23o 35’ N; 580o 09’ E). The soil has a loamy texture down to 300 mm depth, with clay content of about 21%, increasing with depth. The average maximum temperature is 45 C in summer and 28 C in winter. The average annual rainfall is 67 mm and soil organic matter in the experimental site was 1.34 %.

Treatments

Four soil water contents and four external loads were used in a complete factorial experiment as follows: soil water - air dry (AD), 50% field capacity (50% FC), field capacity (FC) and saturated (Sat). External loads - no load (L0), small tractor (L1), medium tractor (L2) and large tractor (L3). The small tractor was a Massey Ferguson 240 (2638 kg weight), the medium tractor was a Massey Ferguson 5340 (3912 kg) and the large tractor was a Ford Backhoe 655D (6964 kg). The tyres used in this experiment were diagonal ply, except for the large tractor rear tyres, which were radial.

Results and discussion

Soil bulk density

The original air-dry soil bulk density increased with soil depth: 1.34, 1.42 and 1.49 Mg/m3 for the soil depths 0-100, 100-200 and 200-300 mm (Table 1). The bulk density stayed almost constant, regardless of the soil water unless external loads were applied, and likewise was almost constant for all AD treatments regardless of the applied external loads. However, when load was applied together with water, there was a significant increase in bulk density for all soil water levels above AD treatments.

External load and soil water interaction

The interaction between soil water and external load was insignificant for all soil depths. However, it showed a similar pattern as for soil strength and water infiltration rate where bulk density at a given load increased as water levels increased and likewise bulk density increased at a given soil water as external loads increased. Figure 1 shows the interaction for the 0-100 mm soil depth only as similar interactions occurred for the 100-200 and 200-300 mm soil depths.

Table 1. Soil bulk density for the 0-100, 100-200 and 200-300 mm depths at four water contents and four external loads.

 

Bulk density (Mg/m3)

Depth (mm)

ADL0

ADL1

ADL2

ADL3

100

1.34

1.35

1.37

1.36

200

1.42

1.44

1.43

1.44

300

1.49

1.48

1.48

1.49

50%FC L0

50%FCL1

50%FCL2

50%FCL3

100

1.34

1.43

1.45

1.49

200

1.42

1.46

1.50

1.53

300

1.48

1.47

1.48

1.51

FCL0

FCL1

FCL2

FCL3

100

1.33

1.48

1.53

1.55

200

1.43

1.56

1.58

1.59

300

1.49

1.57

1.59

1.60

SatL0

SatL1

SatL2

SatL3

100

1.34

1.53

1.56

1.64

200

1.43

1.57

1.58

1.63

300

1.49

1.56

1.58

1.64

F values for the 0-100 and 100-200 mm were highly significant (<0.001) while that of 200-300 mm were highly significant (<0.001) for soil water and significant (0.019) for the external load. L.S.D. for 0-100 mm was 0.057, for100-200 mm was 0.053, and for 200-300 mm was 0.043.

Figure 1. Soil bulk density for the 0-100 mm soil layer as affected by the interaction between external loads and soil moisture treatments.

Soil strength

The original averages of the soil strength were 1.14, 1.70 and 2.38 MPa for the 0-100, 100-200 and 200-300 mm soil depths (Figure 2). Thus, soil strength in the field increased with increasing soil depth similar to the pattern of increasing soil bulk density. The differences between treatments were highly significant for all depths and for both the soil water and external load treatments (P<0.001, see Figure 2). For the AD treatments, the increases in soil strength ranged from 3 to 25% and the effect of external load decreased with increasing soil depth. For soil water treatments above AD, that is 50% FC, FC and Sat. treatments, the increases in soil strength ranged from 2 to 44% for the 50% FC, 2 to 99% for the FC and 3 to 169 % for Sat. treatments.

Fig. 2. Soil cone index (MPa) for all treatments and for 0-100, 100-200 and 200-300 mm soil depths. Vertical bars indicate l.s.d. (P = 0.05).

External load and soil water interaction

There was a highly significant interaction (P<0.001) for 0-100 and 100-200 mm soil depth and significant interaction (P = 0.05) for the 200-300 mm soil depth between soil water and external load as shown in Figure 3. When load was applied together with water, there was an increase in soil strength, which means that the increase in soil strength at a given load treatment was greater at higher water levels and likewise the increase in soil strength at a given soil water treatment was greater at higher external loads.

Soil water infiltration

Figure 3. Water infiltration rate for the 0-100 mm soil layer as affected by the interaction between external loads and soil moisture

Also, infiltration rate decreased with increasing external load and soil water. For example, the infiltration rate for the FC treatments decreased by 41, 56 and 72% when soil loads increased from L0 to L1 to L2 then to L3, while infiltration rate for the L2 treatment decreased by 70, 71 and 78 % when soil water increased from AD to 50% FC to FC and then to Sat. The interaction between soil water and load treatments was significant (P = 0.001). Figure 3 shows the effect of the interaction between external load and soil water on soil water infiltration rate. For AD treatments, the infiltration rate was proportional to the soil strength and decreased with increasing soil strength regardless of the external load.

Tyre inflation pressure and surface area contacts

The rear tyre weight of each of the three tractors was higher than that of the front tyre because of the heavy weight of the hydraulic assembly and other attachments. An exception to this was for the small tractor MF 240, which had a heavier front weight than that of the rear because a front-end loader assembly was attached to the front of the tractor. The front tyre compaction of the tractors was 340, 102 and 235 kPa for the Backhoe 655D, MF 5340 and MF 240 tractors respectively, whilst the corresponding rear tyre compactions for the same tractors were 378, 97 and 89 kPa (Table 2)

Table 2. Infilation pressure, tyre and tractor/ground surface area contact, tyre weight, tyre compaction and maximum compaction per tractor (Max compaction) for the large tractor Ford Backhoe 655D, medium tractor MF 5340 and small tractor MF 240.

 

Tractor

Inflation pressure

Tyre/ground surface area contact

Tyre weight

Tyre compaction

Max compaction

 

kPa

kg/cm2

cm2

kg

kg/cm2

kPa

kPa

Backhoe 655D

             

Front tyre

193

2.6

237

822

Front 3.47

Front 340

378

Rear tyre

207

0.7

710

2740

Rear 3.86

Rear 378

 
               

M F 5340

             

Front tyre

69

1.0

1023

1064

Front 1.04

Front 102

102

Rear tyre

69

1.2

1173

1162

Rear 0.99

Rear 97

 
               

MF 240

             

Front tyre

193

2.2

315

754

Front 2.39

Front 235

235

Rear tyre

69

2.5

639

578

Rear 0.90

Rear 89

 

When the maximum surface soil compaction per tractor is calculated, it shows that the large tractor Backhoe 655D has the highest compaction at 378 kPa, the second highest was the small tractor MF 240 at 235 kPa and the third or the lowest tyre compaction belonged to the medium tractor MF 5340 at 102 kPa.

Conclusions

Both soil water and external load significantly affected soil compaction as measured by soil bulk density, soil strength and soil water infiltration rate. There were significant interactions between soil water and external loads in increasing soil strength and soil water infiltration rate but the interaction was not significant for soil bulk density. The combined impact of soil water and external load on soil compaction demonstrates that soil should be worked at the lowest possible soil water contents and exposed to a minimum external load when conducting farm operations. Tyre load and inflation pressure are key factors and should be kept as low as practically possible during farm operation.

Acknowledgement

This study was supported by the Commonwealth of Australia through an Endeavour Research Fellowship granted to the senior author, Sultan Qaboos University (SQU)- College of Agriculture and Marine Science, and the Agricultural Systems Programme of the Department of Agriculture and Food Western Australia.

References

Batey, T 2009. Soil compaction and soil management - a review. Soil Use and Management 25, 335-345

Hamza, MA and Anderson, WK 2003. Responses of soil properties and grain yields to deep ripping and gypsum application in a compacted loamy sand soil contrasted with a sandy clay loam soil in Western Australia. Australian Journal of Agricultural Research 54, 273–282.

Hamza MA and Anderson WK 2005. Soil compaction in cropping systems. A review of the nature, causes and possible solutions. Soil and Tillage Research 82, 121-145.

Soane, B.D., Van Ouwerkerk, C. Eds., 1994. Soil Compaction in Crop Production, Developments in Agricultural Engineering. Series, vol. 11. Elsevier Science, Amsterdam, The Netherlands, pp. 662.

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